1. Field of Invention
This invention relates to a process for the conversion of oxygenates to hydrocarbons using small pore non-zeolitic molecular sieve catalysts. More particularly, this invention relates to a catalyst composition, a method to prepare such a catalyst, and a process to use such a catalyst in a process for conversion of oxygenates to olefins using silicoaluminophosphate molecular sieve catalysts which have been incorporated with certain alkaline earth metals either during or after the synthesis of the molecular sieve.
2. Background Art of the Invention
Olefins have traditionally been produced through the process of petroleum cracking. Because of the potential limited availability and high cost of petroleum sources, the cost of producing olefins from such petroleum sources has been steadily increasing. Light olefins such as ethylene serve as feeds for the production of numerous chemicals.
The search for alternative materials for the production of light olefins such as ethylene has led to the use of oxygenates such as alcohols, and more particularly to methanol and ethanol or their derivatives as feedstocks. These and other alcohols may be produced by fermentation or from synthesis gas. Synthesis gas can be produced from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal wastes, or any organic material. Thus, alcohol and alcohol derivatives may provide non-petroleum based routes for hydrocarbon production.
It is well known in the prior art to convert oxygenates to olefins by contacting the oxygenate with various types of catalysts. Large, medium, and small pore, zeolitic and non-zeolitic, molecular sieve catalysts may be used.
It is also well known in the prior art that molecular sieves of various pore diameters and compositions have been treated by addition of alkaline earth metals to improve catalyst performance for use in various applications. It is also well known that when comparing the performance of two catalysts, even if every physical parameter of each of the catalysts is the same, that if the two catalysts have a different composition, then one cannot predict based on the performance of one catalyst, how the second catalyst will perform, U.S. Pat. No. 4,752,651, col. 2, lines 31-68. So even if a particular alkaline earth metal has been added to one type of catalyst for a particular use, it does not mean that the same metal will have the same beneficial effect on the performance of the second catalyst.
Even though the art teaches the use of some of the alkaline earth metals to improve the performance of large, medium and small pore zeolites, it fails to teach the use of all such alkaline earth metals, including strontium, calcium, and barium, to improve the performance of non-zeolitic molecular sieve catalysts with diameters of less than about 5 Angstroms for the use in oxygenate conversion.
U.S. Pat. No. 4,752,651 teaches the modification of small pore non-zeolitic molecular sieve catalysts using the alkaline earth metals of beryllium and magnesium. However, the prior art fails to teach and/or enable either the incorporation of the alkaline earth metals of strontium, calcium, and barium into small pore molecular sieves or the in situ process of such metals into such a catalyst for the use in oxygenate conversion.
This failure to teach may be due to larger ionic radii of the cations with the higher atomic numbers in Group IIA. For example, beryllium and magnesium each have a size of 0.31 and 0.65 Angstroms, respectively. This is to be contrasted with the larger sizes of calcium, strontium, and barium with ionic radii of 0.99, 1.13, and 1.35 Angstroms, respectively. Based on this size difference, one of ordinary skill in the art would not think that these larger radii ions could be used as effectively in modifying a small pore catalyst. Even though all of these radii are less than 5 Angstroms, it is well known that the ions exist in the solvated form with the solvent molecules attached. Therefore, even though the metal ion has a radius of less than 5 Angstroms, that in the solvated form, the effective radius will be much larger.
Meanwhile, JP94074134 (JP01051316) discloses an in situ process which appears to be a method to make a small pore aluminophosphosilicate containing any one of the alkaline earth metals which is useful in an oxygenate conversion process. However, upon a close reading of the disclosure, this patent actually teaches the use of a medium pore catalyst, such as ZSM5, and not a small pore catalyst, such as SAPO-34, for oxygenate conversion. The disclosure focuses on how their catalyst is unique compared to a conventional ZSM-5. For example, their catalyst has a pore diameter of 5 to 6 Angstroms and an adsorption volume that is similar to that of common ZSM-5 type zeolites. The x-ray pattern of their material is similar to medium pore sized ZSM-5 and not similar to that of small pore sized SAPO-34. Their catalyst is described as a novel zeolite that has a pore diameter that is between large diameter zeolites, such as faujasite X and Y types, and small diameter zeolites, such as erionite and offretite which further distinguishes their catalyst from a small pore molecular sieve.
Therefore, based on the teachings of the prior art, it is surprising to learn that the alkaline earth metals of strontium, calcium, or barium can be successfully added to a small pore non-zeolitic molecular sieve for enhancement of the performance for such a catalyst for use in the oxygenate conversion process.